Surface treatment of molybdenum metal



United States Patent 3,011,923 SURFACE TREATMENT OF MOLYBDENUM METALCharles 0. Cofier, Livermore, Califl, assignor to the United States ofAmerica as represented by the United States Atomic Energy Commission NoDrawing. Filed Feb. 19, 1960, Ser. No. 10,002 4 Claims. (Cl. 134-3) Thisinvention deals with a process of removing surface scale frommolybdenum, and in particular of removing oxidic scale.

Molybdenum metal has a great many uses in the industry because of itshigh melting point and its great strength at temperatures up to about870 C. It has been used for chemical apparatus which have to be operatedat such high temperatures, and it now also finds extensive use asstructural material of nuclear reactors, and in particular as thecladding material for fuel elements of reactors using fast neutrons. Thelatter use is possible because of the comparatively low neutron-capturecross section for fastand intermediate-energy neutrons of molybdenum andalso because of the inertness of molybdenum to alkali metals which areused as coolant in such reactors and there come in contact with thecladding of the fuel elements.

For the use of molybdenum in neutronic reactors just described it isparticularly important that the surface of the molybdenum be free fromoxide and any other compounds that might have formed on the surface;this is necessary because the oadde of the surface scale reacts withsodium and forms sodium oxide, 2. highly undesirable reaction.

Molybdenum has been descaled heretofore by immersion in a molten mixtureof sodium hydroxide and sodium nitrite in a proportion of about 70 and30% by weight, respectively, at a temperature of between 260 and 370 C.This descaling bath requires a chemical aftertreatment for cleaning thesurface from the adhering descaling material; this has been accomplishedby immersion in various acids.

The above-described method has a number of drawbacks. The use of moltensalts at the temperature necessary and described above is ratherhazardous. Furthermore, the molybdenum is attacked by the sodiumhydroxide-sodium nitrite bath, and a metal loss of up to three mils foran immersion of five seconds has been observed. Furthermore, some of theafter-treatments were found not to be too satisfactory, because theyalso often attacked the molybdenum; this, for instance, was found to bethe case for an after-treatment solution of a 25% nitric acid containing2% by volume of hydrogen fluoride. Sandblasting for the after-treatmentwas found not to be feasible for parts having a complex surfacestructure.

It is an object of this invention to provide a process for the surfacetreatment of molybdenum articles having a scale on their surface whichrequires a comparatively low temperature.

it is another object of this invention to provide 'a process for thesurface treatment of molybdenum articles having a scale on their surfacewhich does not useany molten salts and thus does not involve the samehazards.

3,011,923 Patented Dec. 5, 1961 ICC It is finally also an object of thisinvention to provide a process for the Surface treatment of molybdenum.articles having a scale on their surface which uses bath compositionsthat do not attack the molybdenum metal.

These objects are accomplished by immersing the molybdenum article to betreated in a mixture of sodium hydroxide and potassium permanganatesolutions, rinsing the article with water, and immersing the article inan aqueous acid bath.

It was found that the oxidic scale on the molybdenum articles isconverted, by the sodium hydroxide-permanganate solution, to substancesthat can be easily removed by a pressure-spray rinse; that is incontradistinction to the oxidic scale originally formed on themolybdenum which adheres firmly to the surface and cannot be removed bymerely rinsing; After the water rinse a black smut still remains on thesurface of the molybdenum article. The smut, however, can be readilyremoved by immersion in a second bath containing one or several acidsthat do not attack the molybdenum. For instance,

a mixture of sulfuric, hydrochloric and chromic acids has givenexcellent results, and the preferred proportion of this acid mixture is15% by weight of sulfuric acid, 15% by weight of hydrochloric acid, byweight of water and from 60 to 100 grains of chromic acid per liter ofthe mixture. The temperature of the first, the descaling, bath rangespreferably between 65 and C., While for the second smut-removing bathroom temperature, about 25 C., is satisfactory.

Instead of mixing sodium hydroxide and potassium permanganate for thepreparation of the descaling bath, sodium hydroxide may also be fusedwith manganese dioxide and the melt thus obtained can then be dissolvedin water. A solution thus prepared is just as satisfactory for thepurpose of this invention as is a sodium hydroxidepermanganate mixture.

In the following an example is given to illustrate the process of thisinvention.

Example A piece of molybdenum metal which had a heavy coat of scale onits surface was immersed in a solution containing 10% by weight ofsodium hydroxide, 5% by weight of potassium permanganate and 85% ofwater; the temperature of the bath was about 75 C. Immersion was carriedout for seven minutes after which the molybdenum article was withdrawnand thoroughly rinsed with tap water. Thereafter the molybdenum articleWas immersed for eight minutes in a solution containing 15 by weight ofsulfuric acid, 15 by weight of hydrochloric acid, 70% by Weight of waterand eight grams of chromic acid per milliliters. of acid solution. Thesolution had a temperature of 27 C. After withdrawal of the molybdenumarticle from this acid bath its surface was found to be completely freefrom scale, smut and had a chemically clean metallic appearance.

It will be understood that this invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

l. A process of removing scale from molybdenum articles comprisingimmersing said articles in a descaling bath containing a mixture ofsodium hydroxide and potas- 3 sium permanganate and being at atemperature of between 65 and 85 C.; rinsing the articles with water;and immersing said articles in an acid bath consisting of sulfuric,hydrochloric and chromic acids.

2. The process of claim 1 wherein the descaling bath 5 contains byweight of sodium hydroxide, 5% by weight of potassium permanganate and85% by weight of water.

3. The process of claim 2 wherein the acid bath contains about by weightof sulfuric acid, about 15 by weight of hydrochloric acid, of water andfrom 60 to 100 grams of chromic acid per liter of the mixture.

4. The process of claim 3 wherein the acid bath contains grams ofchromic acid per liter of the mixture.

References Cited in the file of this patent UNITED STATES PATENTS Re.22,887 Spence June 3, 1947 2,041,331 Gann et al May 19, 1936 2,883,311Halpert Apr. 21, 1959 FOREIGN PATENTS 713,219 France Aug. 10, 1931

1. A PROCESS OR REMOVING SCALE FROM MOLYBDENUM ARTICLES COMPRISINGIMMERSING SAID ARTICLES IN A DESCALING BATH CONTAINING A MIXTURE OFSODIUM HYDROXIDE AND POTASSIUM PERMANGANATE AND BEING AT A TEMPERATUREOF BETWEEN 65 AND 85*C., RINSING THE ARTICLES WITH WATER, AND IMMERSINGSAID ARTICLES IN AN ACID BATH CONSISTING OF SULFURIC, HYDROCHLORIC ANDCHROMIC ACIDS.